My very basic understanding is:

They wanted android developers that used google play services to basically be able to submit the same app to the amazon/fire store (without major revisions), so they reverse engineered the framework used by Google for api/hooks between the apps/apks and the "play-services"/OS levels.

Sort of spoofed the environment to prioritize compatibility in order to make it as easy as possible to grow the Amazon app store.

People don't realize that despite Android being nominally 'Open Source', the closed source Google layer on most phones makes it very difficult to exclude Google entirely from the picture and have a user friendly phone environment (both end-user and app-developer/playstore-user).

Basically only Amazon and China had resources to counter it directly with Android, or you could drop that layer and go the less user friendly route of st like AOSP 'pure' phones.